Wireless communication systems are utilized as network systems that excel in mobility and have proliferated at a rapid pace due to improvements in transmission speed over wireless communication zones, the spread of mobile terminals and the appearance of applications suited to mobile communication. In particular, WLAN systems using radio waves in the 2.4-GHz or 5-GHz band have come into widespread use as schemes for wirelessly connecting computer equipment over comparatively short distances indoors or on-premises. Technical specifications for such schemes have been defined, for example, by the IEEE 802.11 family of standards.
Further, there is growing demand for machine-to-machine wireless communication for connecting not only computer peripherals but also printers and mobile telephones to consumer equipment such as digital still cameras and digital video cameras. At present, such equipment is generally connected by wire cables such as those for use with USB or IEEE-1394, but wireless connection methods are also under consideration as methods by which users can connect these devices in simple fashion.
Unlike WLAN, machine-to-machine wireless communication over very short distances aims to provide a wireless connection within a single person's surrounding environment, considered to be ten meters at most. Such a scheme is referred to as “WPAN” to distinguish it from WLAN. In relation to WPAN, physical and MAC layer specifications have been defined in the ECMA-368 standard by ECMA International, an organization that creates standards, with use being made of UWB (Ultra-Wideband) communication. A wireless USB standard has also been defined as a protocol that operates under ECMA-368.
In order to prevent so-called “frame collision”, which is a state in which multiple wireless terminals transmit wireless frames simultaneously in a WLAN or WPAN system, the timing at which each wireless terminal accesses the wireless media is controlled. What determines the method of control is the MAC (Media Access Control) protocol. Although various schemes exist for the MAC protocol, they typically can be classified into two schemes, namely asynchronous data transfer and synchronous data transfer.
Generally, in asynchronous data transfer, a terminal that has acquired the right to access the media performs data transmission in accordance with the CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) protocol. If the terminal at the destination of this data receives the data correctly, the terminal generally sends back to the transmitting terminal an acknowledgement response referred to as an “acknowledge frame”. If the data transmitting terminal receives the acknowledgement response, it determines that the data transfer is completed. Conversely, if the transmitting terminal does not receive the acknowledgement response, it determines that data transfer failed and attempts to re-transmit its data upon elapse of a fixed period of time.
Thus, in asynchronous data transfer, it is possible to transfer data to a destination terminal reliably. However, the amount of delay involved in the data transfer varies depending upon failure to acquire media-access privileges or re-transmission of data. Such variation in the amount of data involved in data transfer is referred to as “delay jitter”.
It should be noted that since an application that transfers voice or a moving image requires synchronism or isochronism for a data transfer, asynchronous data transfer, which cannot easily provide such latency bounds on delay jitter, is not suitable for such an application.
With synchronous data transfer, on the other hand, each terminal performs the data transmission in a time slot allocated to the terminal as by a TDMA (Time Division Multiple Access) protocol. With such a protocol, each terminal is capable of acquiring the data transmission privilege at fixed periods and the delay jitter imposed upon the data transfer is small as a result. Accordingly, synchronous data transfer is well-suited for real-time data transmission having minimum delay requirements.
Further, with synchronous data transfer, it is required that delay jitter in a data transfer be kept within fixed limits. As a consequence, no acknowledgement response is exchanged between sending and receiving terminals and re-transmission processing is not executed when data fails. This means that with synchronous data transfer, there may be instances where data packets are lost, depending upon the state of communication.
However, in the transmission of voice or moving images, a characteristic of such communication is that the highest priority is always given to the maintenance of an average transmission speed and average delay. The application protocol therefore is designed so as to allow packet loss to some extent.
Thus, whereas synchronous data transfer is suitable for applications using that require delay bounds, as in the case of voice and moving images, application data can vanish due to packet loss. Such data loss is a drawback in that it can cause reproduced voice to be interrupted and degrade application quality.
In order to solve these problems encountered in synchronous data transfer, a repeater system in which identical data is transferred redundantly multiple times has been considered as a highly reliable communication system for suppressing the occurrence of data loss while maintaining synchronism. Such a repeater system will now be described.
FIG. 16 is a diagram illustrating an example of the configuration of a repeater system. This system comprises a control station 11 serving as a source that generates synchronous data such as voice or moving images, and four dependent stations 12 to 15 that are the destinations of synchronous data.
FIG. 17 is a diagram illustrating media access timing of the repeater system. This system employs TDMA as the access protocol, and the media access timing is managed in units of a fixed time period referred to as a superframe 21. The superframe 21 has been divided into time slots 31 to 35 in which the control station 11 and dependent stations 12 to 15, respectively, transmit data. In superframe 21, time slot 31 is the time over which the control station 11 transmits data, and the time slots 32 to 35 are the times over which the dependent stations 12 to 15, respectively, transmit data successively.
FIG. 18 is a diagram illustrating a wireless frame transmitted in each time slot of the superframe. Initially, the control station 11 broadcasts a broadcast frame 41 to all dependent stations in time slot 31 of the superframe. The broadcast frame 41 holds synchronous data intended for each of the dependent stations.
FIG. 19 is a diagram illustrating the frame format of the broadcast frame transmitted by broadcast. The broadcast frame 41 includes a field 51 in which an ISO_DATA1 field is placed. The field 51 contains the body of the synchronous data intended for dependent station 12. Similarly, fields 52 to 54 contain synchronous data intended for dependent stations 13 to 15, respectively. A CHECKSUM_FRAME field is placed in a field 55. By examining the checksum, the station that has received the broadcast frame 41 detects any bit error that has occurred in the broadcast frame 41. It should be noted that each item of synchronous data is assumed to be of fixed length in this example.
FIGS. 20A to 20E are diagrams illustrating directions of data transfer in each of the time slots. FIG. 20A illustrates the manner in which the broadcast frame 41 is transmitted from the control station 11 to each of the dependent stations 12 to 15 in the first time slot 31. Upon receiving the broadcast frame 41, each of the dependent stations 12 to 15 decides its own transmit timing by referring to a predetermined transmission sequence using the moment in time at which the broadcast frame 41 is received as a reference. From within the synchronous data contained in the broadcast frame 41 received, each of the dependent stations 12 to 15 utilizes the data intended for itself as application data. At the same time, the dependent stations 12 to 15 store the entirety of the broadcast frame 41 internally in order to relay it to another dependent station.
Upon receiving the broadcast frame 41 and deciding its own transmit timing, the dependent station 12 transmits a relay frame 42 in time slot 32 as a duplicate of the broadcast frame 41 received from the control station 11. This is illustrated in FIG. 20B. The other dependent stations 13 to 15 receive and store the relay frame 42. Similarly, in time slot 33, the dependent station 13 transmits a relay frame 43 as a duplicate of the broadcast frame 41 and the other dependent stations 12, 14 and 15 receive this relay frame 43.
Thus, the broadcast frame 41 transmitted from the control station 11 in time slot 31 is relayed from the dependent stations 12 to 15 in the time slots 32 to 35, as illustrated in FIGS. 20B to 20E. Further, relay frames 42 to 45 that are relayed have a format identical with that of the broadcast frame 41. Accordingly, as far as each of the dependent stations is concerned, there is an opportunity to receive the same synchronous data a total of four times as the broadcast frame from the control station 11 and the relay frames from the other three dependent stations.
Data is thus transferred multiple times in this system. As a result, even if a dependent station cannot receive a broadcast frame from the control station correctly, it can acquire the synchronous data intended for its own station from the relay frame transmitted from another dependent station. With this system, therefore, even if an event such as an interruption in the communication path occurs, a data transmission is completed within one superframe. This makes it possible to maintain the synchronism of the data transfer.
It should be noted that a technique for transmitting broadcast data to a destination node reliably has been proposed in the specification of Japanese Patent Laid-Open No. 2007-266876.
In the repeater system described above, packet loss is suppressed and highly reliable synchronous data communication achieved by relayed transmission of the same data between dependent stations. However, when this method is considered from the standpoint of efficient utilization of the frequency band, it is apparent that the same data is transmitted repeatedly and redundantly five times. Another aspect of this method, therefore, is that the communication band is utilized wastefully.
By way of example, if a certain dependent station is capable of receiving the broadcast data from the control station correctly, then repeated transmission in subsequent time slots is unnecessary communication and the communication band is consumed wastefully. In such a redundant repeater system, the communication band is essentially occupied by repeated transfer of data and it is not possible to transfer a greater amount of data.
As a result, a problem with the prior art is that it is difficult to optimize utilization of the communication band and increase the amount of data transferred in the system overall.